Centralized control circuits for flat type switches



Dec. 22, 1953 J. H. VOSS CENTRALIZED CONTROL CIRCUITS FOR FLAT TYPE SWITCHES Filed March 8, '1950 LINE FBDER 6 Sheets-Sheet 1 INVEN TOR. JOHN H. V055 ATTORNEY a Sheets-Sheet 2 a o m w a u m. w M -i, M% MW. WW3 W 1 J. H. voss CENTRALIZED CONTROL CIRCUITS FOR FLAT TYPE SWITCHES Mkj l I Dec. 22; 1953 Filed March 8, 1950 INT GEN

l l 5 TE (HIGH) INVENTOR. JOHN H. VOSS ATTORNEY m =4 m w m 4 T ------l--- 3 I J w Dec. 22, 1953 J. H. voss CENTRALIZED CONTROL CIRCUITS FOR FLAT TYPE SWITCHES Filed March 8, 1950 H 5 CONTROL UNIT m In 4 Q 9 Q 3 M l 2 5 4 1m Q m MJ III I I A. I ll Al II I l I HI I: :i w 45 6 O 0 I I 8 l 6 .H m 4 4 4 m 4 Q 0 4 4 O 9 J 4 4 4 5 |I III IIIII II I I I l III III I III f x 5. M 4 4 9 6 @7896 7896 mw mw m 5 6 WW .8 %U %2 3 3334 M &5 3 3 3 O m 33 3 33 33 3 3333 n r (I w rim N {J 4 4 4 l I 5 460 fl I- INVENTOR. JOHN H. voss 2% a ATTORNEY Patented Dec. 22, 19 53 CENTRALIZED CONTROL CIRCUITS FOR FLAT TYPE SWITCHES John H. Voss, Rochester, N. Y., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application March 8, 1950, Serial No. 148,284

22 Claims. 1

This invention relates in general to automatic telephone exchange systems, but is more particularly concerned with the centralized control system for a train of flat type switches.

An object of this invention is to provide a common control unit which controls the operation of the associated flat type selector and connector switches, and whereby relays at the selector and connector switches are completely eliminated.

A feature of the present invention is that considerable economic advantage is had by controlling selectors and connectors from a single relay unit.

Figures 1, 2 and 3 show a diagrammatic representation of one system utilizing the present invention and illustrating an arrangement whereby each selector and connector switch uses a four conductor bank. Figures 1 and 2 illustrate diagrammatically the centralized control unit; Figure 3 illustrates diagrammatically the selector and connector switches.

Figures 4, and 6 show a diagrammatic representation of another system utilizing the present invention, and illustrating an arrangement whereby the selector uses a six conductor bank and the connector uses a four conductor bank. Figures 4 and 5 illustrate diagrammatically the centralized control unit; whereas Figure 6 illustrates diagrammatically the connector and selector switches.

Figure 7 is a P. B. X hank contact grouping arrangement for the EC and C conductors of a group of private branch exchange lines in the system shown in Figures 1, 2 and 3.

Figure 8 is a P. B.- X bank grouping arrangement for the E8 and C conductors of a group of private branch exchange lines in the system shown in Figures 4, 5 and 6.

Figure 9 illustrates the grouping arrangement for the respective figures illustrating the several K systems.

Referringnow to Figures 1, 2 and 3, line finder 34 is of the type well-known in the art. Substation A includes a dialing mechanism and is also well-known in the art. Substation A is a substation of the calling party. The circuit herein disclosed may be used with the flat type switches which are well known in the art, but are preferably used with the flat type switches described in a pending application, Serial No. 792,510 filed on December 18, 1947, by Hans Sengebusch, now Patent 2,578,095, issued December 11, 1951, in which the operation of the release magnet permits the switch to remain ofi-normal and whereby the switch releases when the release magnet is deener- 2 gized. The selector switch has a common drive magnet 209, which steps the wipers of the fiat type switch in both the primary andsecondary movements. The shift magnet 211E operates to shift the operation of the switch from the primary to the secondary movement. Release magnet 2 l t operates when the relay 23 operates on seizure, and remains operated until the switch is to be released. The selector switch releases in response to deenergization oi the release magnet 2 i 9. The connector switch also has a common drive magnet 239, which steps thefiat type switch wipers in both the primary and secondary movements.

-The shift magnet 22d of the connector operates to shift the operation or the switch from the primary to the secondary movement. Release magnet Z it of the connector switch operates the .moment the connector is seized bythe selector switch, and remains operated until the switch is to be released. The connector switch is released in response to the deenergization of the release magnet 24%. The centralized control relay unit in Figs. 1 and 2 controls the operation of both the associated selector and connector switches.

. In this specification the expression control relay unit means a group of relays which precede a train of switches and exerts control over a, plurality of contacts to bring about successive operations in the switches taken into use whereby the. number of control relays individual to the primary movement.

line or the first line switches may be reduced.

Relays 68 and ltli are the testing relays for both the selector and connector switches and the switches are of the battery searching type which must find negative battery potential in orderto switch through. The first series of dial pulses operates the selector switch in its primary movement and the selector hunts automatically in its secondary movement. The second series of dial pulses step the seized connector switch in the The third seriesof, pulses steps the connector switch to either a subscribers in a P. B. X group. If a first line in a P. B. X group is connected with the connector and such line is busy, an automatic stepping circuit is completed to cause the connector to hunt for an idle line of the P. B. X group. Relay F29, included in a self-interrupting circuit, controls a stepping circuit to cause the selector to operate in its secondary movement to hunt for an idle trunk and also cause theconnector to operate in its secondary movement to hunt for an idle trunk line in the P. B. X group.

In Fig. 7 the fEC bank contact of thefirst line in the P. B. X group is shown connected to generator GEN. to provide alternating current. The intermediate EC bank contacts of the intermediate lines are left blank while the EC bank contact of the last line in the P. B. X group is connected to battery through a resistance.

In Fig. 8 the EC bank contact of the first line in this P. B. X group is shown connected to direct ground. The intermediate EC bank contacts of the intermediate 1?. B. X lines are connected to ground through a resistance while the last EC bank contact of the last line in the P. B. X group is left blank.

In more detail, when the. calling subscriber at substation A raises his handset, not shown, from the cradle, line relay If] is seized through the associated line under 3 in the Well-known manner. Relay it] operates over the subscribers loop. Contacts [2 make to operate slow-to-releas relay 2B. Relay 26 operates and contacts 24 make to ground the C lead to hold the line finder and the line equipment. Contacts 26 make'to put ground on conductor it to operate the release magnet 21;; of the selector switch to permit the selector switch to step under th control of the drive magnet 299 of the selector switch. Contacts 23 close to prepare the pulsing circuit to permit the selector switch to step under the control of the drive magnet 269 in response to the dialing pulses from substation A and also to prepare a circuit to operate relay 3c. Contacts 2i close to prepare the locking circuit for relay Hit of the control unit; contacts 22 close to prepare the locking circuit for ring-cut-ofi relay Hi; contacts 2t put ground on conductor 1 to prepare the locking circuit for relay I59.

The callin party at substation A dials a series of digital impulses corresponding to the number of the called party to interrupt the circuit to relay it for repeating impulses over the following path: ground, contacts ii, contacts 23, conductor 5, contacts ltl, contacts ill, contacts 143, contacts Hi3, conductor 186, S. O. N. contacts 213 and drive magnet 269 to battery. Slow-to-release relay 3t operates during each series of impulses over the following path: ground, contacts H, contacts 23, relay to and battery.

Relay to operates and contacts 3| make to connect relay to in multiple with relay 2c; contacts 33 break to disconnect relay no from the C Wiper 225 and contacts 32 break to open a circuit to disconnect relay I59 from the positive talking conductor. Relay til being a slow-to-operate relay does not operate during the receiving of impulses repeated by relay Iii, but does operate at the completion of each series of pulses before slow-to-release relay 3c releases.

make to operate relay I30 in series with the shift magnet 2GB over the following path: battery, relay !30, conductor l5, contacts 43, contacts 5i, conductor I4, contacts IBI, contacts H2, contacts M5, contacts Hi5, conductor 16?, S. O. N. contacts 2M, P. O. N. contacts 2l5, shift magnet 2530 and ground. It is to be noted that shift mag- ..net 2% is operated in series with relay I after idle connector in the manner set forth in the cited The first series of impulses are transmitted to the drive magnet 2&9 of the selector by grounding the back contacts of relay it over the previously described path. ihe drive magnet 289 steps the selecto wipers 234, 235, 2!!! and 2.25 to the bank level selected in accordance to the number of digital impulses in the first digit. When the selector wipers take the first step, P. O. N. contacts 2l5 prepare a circuit to the shift magnet 260 of the selector switch and contacts 2H3 open a circuit to the cam switch as well as disconnecting negative battery potential through resistance R from from conductor its to prevent the operation of test relays, and I60 when relay 59 operates. After the first series of impulses are repeated and before relay 38 releases, relay 50 operates over the following path: battery, relay 50, contacts 3i, contacts i2 and ground. Contacts I 5| Sengebusch application. Contacts 281 and M33 break to open the circuits to the wipers to prevent interference during rotation thereof. Contacts 287 open the circuit to wiper 2E9 of the selector and contacts 208 prepar a ground connection to the all trunk busy meter. Relay operated in series with shift magnet 200 and contacts I33 completes a holding circuit to relay 5% to maintain relay to operated. Contacts 134 make to operate pulsing relay I26 for the stepping of the drive magnet 209 in the secondary direction. Contacts 32 make to prepare the pulsing circuit for the drive magnet 209 to step the selector switch in the secondary direction. Contacts l3i break to open the primary pulsing circuit to the drive magnet 2B9. Relay 3|] releases shortly after the first digit.

Relay ifit operates over the following path: battery, relay iil, contacts iS, contacts H33, contacts 22 and ground. The selector is of the battery searching type and when an idle switch is found a high resistance battery is found on the C wiper 225 to operate high resistance relay H38. When pulsing relay i213 operates, contacts i2! make to operate the drive magnet 239 to step the selector switch wipers to the first bank contact in the secondary movement over the following path: ground, contacts E21, contacts i32, contacts lH, contacts 143, contacts W3, conductor E96, S. O. N. contacts M3 or 202, magnet 239 and battery. The switch steps in the secondary movement and S. .0. N. contacts 2E3 and 2% break on the first step. If the connector is busy the bank contact engaged by wiper 225 will have a ground potential thereon or if the wiper 225 fails to find negative battery potential, the test relays Gil and H39 fail to operate. In either case relay 52s in errupts its own circuit at contacts 822 and in case test relay 556 has not operated relay E29 is reoperated on closure of contacts 22 to repeat the above-described operation for operating the drive magnet 2&9, this time over contacts 292 since 5. O. N. contacts 2E3 are now open. This operation is repeated until an idle connector having battery potential connected to its associated bank contact is found or until the ilth bank contact is reached.

In case the test wiper 225 fails to find an idle connector switch, the wipers of the selector step off the last bank contact of the selected group to theirllth position and the can: springs CS are operated. Cam Springs 2553 close a circuit for operating the all trunk busy meter AiB-l/iTR from ground at contacts 288, cam springs 253 and the common all trunk busy meter conductor. Cam springs 2 i? close a circuit for operating the high resistance test relay I60 as follows: ground thru the winding of relay I60, winding of the low resistance test relay 68, contacts 53 and M9, conductor E39, cam springs 2, contacts 295 and thru the high resistance R to battery. Due to the high resistance R relay 68 does not operate but relay its operates and at contacts Nil opens the circuit to relay I30 and shift magnet 2130 to cause their release. At contacts I63, relay I69 opens the circuit to pulsing relay I29 to prevent its reoperation.

The release of shift magnet 299 in the selector switch opens the circuit to stepping magnet 2% at contacts 202 to prevent reoperation of magnet 289. At contacts 264 magnet 2% opens a further point in its own circuit and at 28% opens the circuit to the all-trunk busy meter ATB-MTR. At contacts 265, magnet G opens the circuit to test relay 166 and substitutes ground at contacts 2% to close a shunt circuit around test relay S69 thereby rendering the test relay somewhat slow 1 to release.

Relay as, upon releasing, at contacts i32 opens a further point in the pulsing circuit, at contacts 53% opens a further point in the circuit to pulsing relay Hit, and at contacts I33 opens the circuit to relay 5% which releases. Relay 5% at contacts 53 opens the circuit to test relay its and at contacts t l completes a circuit for operating busy relay its as follows: ground, contacts 286, 22?, conductor 588, contacts MS, E i, 33 and us, and thru the Winding of busy relay I 10 to battery.

Busy relay l is, upon operating, at contacts ill opens a further point in the stepping circuit, at contacts H2 opens a further point in the circuit to shift magnet 23%), and at contacts H3 completes a locking circuit for itself from grounded contacts 27 after which contacts H2 opens its original energizing circuit. At contacts H6 busy relay Hi3 substitutes grounded busy tone source for the disconnected ground at contacts H5. Busy tone is now transmitted to the calling party through the lower winding of line relay i9 and over the calling loop in the well-known manner. In response to receipt of busy tone the calling party replaces his receiver to release the switch train in a manner to be described later. v

Let us assume that the connector connected to wipers 234, 235, 219 and 225 is the first 'idl-e connector engaged by said wipers during its searching operation. C wiper 225 is connected to battery through high resistance 23%, contacts 244 and P. O. N. contacts 229 of the connector.

The system is now prepared for dialling the second digit to step the connector in the primary movement. The second series of pulses from the substation A operates line relay it to repeat these pulses to the drive magnet 23% of the connector switch over the following path: ground, contacts H, contacts 23, conductor 5, contacts 13!, contacts HI, contacts its, contacts i633, conductor I06, contacts 2%, contacts 2! I, wiper 23 5, S. O. N. contacts 226, drive magnet 23ft and battery. Relay 30 again operates over a path previously described and at contacts 32 opens the operating circuits to relays 4B and 39. Drive magnet 230 steps the connector switch wipers in response to the repeated impulses to the selected bani: level.

When the connector switch stepped on the first pulse of the primary movement, P. 0. N. contacts 228 make and P. O. N. contacts 225 break. P. (1N. contacts 228 prepare the circuit to operate the shift magnet 226 of the connector switch. Slowto-operate relay to operated again at the completion of the second series or" digital impulses before relay 3t restores and at contacts 52 opens the operating circuit for relay it. Relay I33 does not operate this time, since shift magnet 22% of the connector is battery connected.

Relay 3t releases, contacts 3! open the operating circuit of relay 5t, and contacts 32 prepare the operating circuit for relay 40. Relay 5!? releases and contacts 52 make to place the lower winding of relay to in series with the shift magnet 229 of the connector over the following path: battery, shift magnet 228, P. O. N. contacts 228, S. O. N. contacts 22?, wiper 2-25, contacts 2 i2, contacts 293, conductor iu'l, contacts 9%, contacts I45, contacts H2, contacts iti, conductor i i, contacts 52, contacts 32, conductor E3, contacts M7. conductor 9, relay to and ground.

Shift magnet 22% operates over the abovetraced circuit and prepares the connector mechanically, as described in the cited application,

. for stepping the wipers are, 2 5?, 2st and 259 High resistance relay I50 operates over the following path: ground, high resistance relay 159, low resistance relay 6%, contacts 53, conductor i9, contacts H59, conductor I69, wiper 225, P. 0. N. contacts 22%, contacts 254, resistance 235 and battery. Relay I68 alone operates due to high resistance 2% and contacts I63 open the operating circuit of pulsing relay I20. Contacts 565 open the circuit to release shift magnet 289 and relay :32. Contacts 563 open the operating circuit to relay E28. Relay I30 releases and contacts l3! make to prepare the pulsing circuit for the primary movement 'for the connector switch. Contacts H32 break to open the pulsing circuit for the secondary movement; contacts 13% break to further open the circuit to the pulsing relay E28; contacts H3 break to release relay 5B. Relay 5t releases and contacts 5! break to further open the operating circuit for relay I38; contacts 53 break to open the testing circuit to relays 6t and IE0, and contacts 52 close to prepare an operating circuit for shift magnet 229 of the connector. Relay Hill releases and contacts i6! prepare the operating circuit to the shift magnet 22!] of the across the bank contacts of the dialled level in response to the third series of digital impulses. Magnet 22!), at contacts 22i and 223, opens the circuit to wipers 2 56 and as; to prevent disturbances as such wipers are passing over the bank contacts of busy called lines, at contacts 222 maintains the pulsing circuit to magnet 23% independent of the subsequent operation of the secondary off-normal contacts S. O. N and at contacts 222 maintains the circuit to shift magnet 220 when S. O. N. contacts 22? open. Relay All operates over the above-traced circuit and contacts 4i prepare an operating circuit to relay Mil.

Contacts 42 complete an alternative circuit between relay to and shift magnet are independent ofcontacts 32 which are opened when relay 2? 7 drive magnet 230 may betraced over the, follow ing path: ground, contacts il,s..contacts;23,;ccnductor 5, contacts l3i, contacts ill, contacts Hi3, contacts 83, conductor lot, contacts 2M, contacts 2i i, wiper 231i, contacts xi'ifidrivernagnet 239 and battery.

The connector switch takes thefirstsecondary step and the S. O. N. contactsiifipandifl break. The shift magnet 22c maintainsa circuit'to the drive magnet 23% at'contacts 222 when S. 0.1 N. contacts 225 open on the firststepinthesecondary movement and atcontacts 22! and lit-pravents interference with the lines over which the wipers pass.

At the end or" the third series of digital impulses sloW-to-o ate relay again operatesbefore relay 3t role... 3 and contacts 53 make to connect the testing relays Ell and lotto the C wiper 255 or the connector to test ioran idle line.

If the called line is idle, battery on C".wiper 2% will operate relay as in serieswith relay 69 over the following path: ground, relay Hill, relay 53, contacts contacts l te, conductor 2E9, wiper 225, contacts contacts 2:35, wiper 243, and through the winding of the caLed line cut-off relay to battery. Relays lfiil and E9 operateand relay to at contacts Bl closes a circuit to operate relay see over the following path: battery, relay i lil, conductor 2, contacts 3i, contacts El and ground.

if the called line is ousy wiper 2 59 encounters a grounded bank contact with the-result that test relays it and till? do not operate. Now when relays L ll and 5% restore, as previously described, busy relay lid operates over the following pat-h: battery, relay iiil, contacts lid, conductor l3, contacts 33, contacts conductor l9, contacts 149, conductor its, wiper 225, contacts 233, contacts 2:25, C wiper 2:29, and the busy ground found on the C lead of the called line.

Relay iii operates and contacts Block relay l i3 through ground on contacts 2?. Contacts 2 l i open the pulsing circuit to drive magnet 23%. Contacts 5 break. to release the shift magnet 229 and relay to. Shift magnet 22% releases when its circuit is opened at contacts H2 toopenfurther points in the circuits for magnetsilso and 22% at contacts 222 and 22d, respectively. At contacts 22! and magnet closes the talking circuit connection with the called line to the control unit which circuit, however, is open at contacts Hi and i it to prevent interference. It will be noted at this time that if the armature springsof contacts l l l and iii were provided with makecontacts an arrangement could easily be added to provide a means for monitoring thebusy connec tion if desired. The release of-relaycc at this time is v *hout function. At contacts H6 busy relay subst 'utcs the busy tone source for ground thereby transmitting busy tone to the calling party. The release of the'switch train is accomplished by the calling party replacing his'receiver in a manner to be described hereinafter.

Assuming that the called line is idle, relays cc and lit operate in a manner previously described and thereby operates relay its by grounding contacts ti in amanner previously described. Relay upon operatingat contacts lti opens the energizing circuit of relay to and shift magnet are which release. Relay nonoperates and locks itself over the following path: battery, relay are, contacts Ml, conductor l, con tacts'ril and round. Contacts it? and contacts Hi5 open to open further points in the circuit of relay-wand the shift magnet 22!]. Contacts: I44

ill

prepares a-point for the ground return ringing circuit andcontact Mil prepares anew circuit for reoperating theback bridge relay 49. Contacts 645 opens the circuit to test relays 69 and IE0 which release-andcontacts Hi8 put ground on the C wiper 259 to busy the called line. Contacts i455 break to open the pulsing circuit and contacts M2 maketo connect the interrupter generator through the winding of relay lilo. Ring back tone, may besent'tc thecalling party in any known'manncr. Magnet 22% released in, a manner previously described to complete ringingthe called party. The ringing circuit may be traced as follows: Interrupted generator. upper winding of ring-cut-ofi relay lilo, contacts I42, H33, .ZOI, 2il, wiper 234, contacts 22! andZ il, wiper2 l3, over the called subscriber loop; back to wiper 2 3?, contacts 2l2 and 223;.wiper 235,,contacts 2 i2, 263, 95 and W: to ground.

When the called party answers slow-to-operate ring-cut-off relay its operates over the following path: ground, contacts hit, its, 263, M2, 223, 2&2, wiper 2 1?, the called line loop, wiper its. contacts 2 H, con acts 22!, wiper 22c, contacts 2, contacts gill, conductor 86, contacts m3,

contacts Hi2, role ill; and the interrupter generator conductorto battery. Relay HES looks at contacts it! over the following path: battery, lower winding of ring cut-on relay 158, contacts llll, conductor 3, contacts 22 and ground. Contacts loll and H55 open theabove-traced ringing circuit and contacts ifi2 and its complete the talking. circuit between the calling and called party. battery is furnished to the called line through the windings of back bridge relay a which energizes over the called subscribers loop and performs no further function at this time. When the calling party replaces his handset relay it releases. Contacts l2 break to release relay 2c. Relay so removes ground from the C lead through contacts .25; thereby permitting the line finder and line equipment to release. Contacts 2i; break to remove ground from the release magnet 22 to to release theselector switch and also to remove ground'irom the release magnet :248 to release .the connector switch. Contacts 25 remove ground from the loclringcircuit of relay use to release relay Contacts 22 remove ground from the locking circuit to release relay see. Contacts A? removes ground from the looking circuit to release busy relay HQ in case the calling party encounters a busy condition. All relays are now released and the system prepared for further operation.

Referring-now to Figure '7, the foregoing .disclosure has assumed thatan individual line has been called. If a P. X group of subscriber lines having a single call number had been called EC wiper zce and C wiper see of the connector would have encountered conditions as shown in Figure '7 iorthe P. BJX group. Let us assume at this point that the connector has been directively stepped in the secondary movement as was previously described to the first line in a P. B; X group. It is to be noted that the connector is a battery searching connector. If the first line in the P. B. X groupis idle, relays Eli and its will operate in series withtheB. C. O. relay of the called line, over the following path: battery, B. C. O. relay of the called line, the called line bani; contact, wiper Zihcontacts 255, contacts 233, wiper 225, conductor lilo, contacts M9, conductor 59, contacts Eirelay fiurelay 66, and ground.

Relay ltll operates and contacts I6l .openthe circuit to relay '40 and shift magnet 220. Relay operates and contacts 0| close to operate relay I 00 before relay 40 releases over the following path: ground, contacts contacts ll, conductor 2, relay I20, and battery.

Relay I operated and locked itself by closing contacts I ii over the following path: battery, relay I00, contacts t ll, conductor I, contacts 2| and ground. Contacts I01 and I ls open further points in the circuit of relay 40 and the shift magnet 220 in a manner previously described. Contacts I00 open the circuit to relays 00 and I00 and contacts I08 put ground on the C wiper 229 to busy the same over an obvious circuit. Contacts I52 make to connect the interrupter generator to the called line through the upper winding of relay I00. Magnet 220 released in a manner previously described to permit ringing the called party when contacts 22I and 223 make.

When the called party answers, slow-to-operate ring cut-off relay Illa is operated as previously described. Relay I00 looks at contacts I!!! and at contacts 502 and I04 completes the talking circuit between the calling and called party.

If, however, the first line in the group is busy relays 30 and I00 will. not operate. The EC wiper 240 will encounter a grounded generator voltage on the first trunk line, as shown in Figure 7. The generator voltage operates alternating current relay 550 over the following path: grounded generator on the EC bank contact, wiper 208, contacts 223, contacts 232, condenser Cl, wiper 2'55, contacts 2I2, contacts 203, conductor I07, contacts I05, contacts I45, contacts I I2, contacts IBI, conductor I l, contacts 42, conductor I3, conductor C2, relay I50, and battery. Relay I50 operates and locks over the following path: battery, relay I50, contacts I00, contacts 55L conductor I, contacts 25 and ground. Contacts I52 make to operate relay I over the following path: ground, contacts I02, contacts I52, relay I30 and battery. Contacts I53 make to operate relay I20 over the following path: battery, relay I20, contacts I53, contacts I53, contacts I22 and ground.

Relay I30 operates and contacts I33 close to maintain relay 50 operated. Contacts I32 close to prepare a pulsing circuit to drive magnet 230 to step the connector in a further secondary movement. Contacts I34 make to add another operating circuit to relay I20.

'Relay I20 operates and contacts I 2| close to step the connector switch in a secondary movement over the following path: ground, contacts I2I, contacts I32, contacts III, contacts I43, contacts I 03, conductor I 00, contacts 20!, contacts 2II, wiper 234, contacts 222, magnet 230 and battery. Drive magnet 230 steps the connector wiper to the next trunk line of the P. B. X group. Relay I20 self-interrupts its operating circuit through contacts I22. The shift magnet 220 of the connector remains operated in series with relay 40. As heretofore explained, the connector is now automatically stepping across the trunks of the P. B. X group in search of battery to stop its rotation. If an intermediate trunk line is idle, relays 00 and IE0 will operate as heretofore explained to stop the rotation of the connector wipers by opening the operating circuit to relay I20 at contacts I 53, and to cause a switch through to the called line as heretofore explained.

If an intermeditae line is not idle the connector wiper 2 38 upon reaching the last trunk line in the P. B. X group encounters battery through a high resistance, such as R3 shown in Figure 7 be* fore the stepping magnet releases. Relay I60 alone operates due to the high resistance R3 over. the following path: battery, high resistance R3, last "EC bank contacts of the P. B. X group, Wiper 248, contacts 233, contacts 23I, wiper 225, conductor I03, contacts I49, contacts 53, relay 60, conductor 28, relay I and ground. Relay I00 operates to prevent further pulsing by opening the locking circuit of relay I50 at contacts I 00 and the operating circuit of relay I20 at contacts I03. Contacts I02 breaks to de-energize relay I30 and contacts IBI opens the operating circuit of relay 40 and shift magnet 220 which release. Relay I 50 releases to open a further point in the circuit of relay I30 at contacts I52 to prevent reoperation of relay I30 after the subsequent release of relay I and at contacts I53 opens a further point in the circuit of relay I20. Relay I30 releases and contacts I33 break to release relay 50 over a path previously described for maintaining the same operated. Contacts I32 opens, the operating circuit to stepping magnet 230. Contacts I34 break to open another point in the operating circuit to relay I20. Magnet 230 releases and opens contacts 23I to open the circuit of relay I60 which releases. The release of relay I50 is without effect at this time because the circuits to relays I20 and I30 are opened at contacts I53 and I52 by the release of relay I50.

If the last trunk line of the P. B. X group is idle, relays 60 and I00 will also operate as before from battery through the B. C. O. relay of the last line in the P. B. X group and through the C connector wiper before relay 50 restores, thereby causing a connection to be made to the last P. B. X line in the P. B. X group in a manner previously described. The circuit to operate relays 00 and I00 is as follows: battery through the B. C. O. relay of the last line in the P. B. X group, wiper 240, contacts 245, contacts 233, wiper 225, conductor I09, contacts I49, contacts 53, relay 50, conductor 23, relay I 00 and ground.

If the last trunk line in a P. B. X group is busy relay IIO operates after relay 50 restores. C connector wiper 249 encounters a ground connection on the C bank contact of the last line in the P. B. X group to operate relay IIO over the following path: ground on C wiper 200, contacts 245, contacts 233, wiper 225, conductor I00, contacts I49, conductor I9, contacts 50, contacts 33, conductor I8, contacts I I4, relay I I0 and battery. Relay I I0 operates to return busy tone to the calling party as previously described. The system releases in a manner heretofore explained in the operation of a call to a direct line.

Referring now to Figures 4, 5, 6 and 8, line finder 304 is of the type well known in the art. Substation A includes a dialing mechanism and is also well known in the art. Substation A is the substation of the calling party. The circuits herein disclosed may be used with the flat type switches whichare well known in the art, but are preferably used with the flat type switches disclosed in the pending application, Serial No. 792,510, filed on December 18, 1947, by Hans Sengebusch, now Patent 2,578,095, issued December 11, 1951, in which the operation of the release magnet permits the switch to remain offnormal and whereby the switch releases which the release magnet is deenergized. The selector switch has a common drive magnet 530 which steps the fiat type switch in both the primary and secondary movements. The shift magnet 500 'of the release magnet 5H3. switch also has a common drive magnet 5H3 ment.

operates to condition the switch for stepping the switch in the secondary movement. Release magnet 518 operates upon seizure of the switch, when relay 3E9 operates, and remains operated until the switch is to be released. The selector, therefore, releases in response to the de-energization The connector which steps the flat type switch in both the primary and secondary movements. The shift magnet 526 of the connector operates to condition the switch for stepping in the secondary move- Release magnet 53% of the connector switch operates the moment the connector is seized by the selector switch and remains operated until the switch is to be released. The connector switch is released in response to the deenergization of the release magnet 53 A centralized control relay unit controls the operation of both the associated selector and connector switches. The centralized control unit is separately located from the aforementioned switches.

Relay is a resting relay for both the selector and connector switches, and the switches are of the battery hunting type. The first series of dialing pulses operates the selector switch for the primary movement after which the selector hunts automatically in its secondary movement to seize an idle connector switch. The second series of dial pulses steps the connector switch in its primary movement. The third series of pulses steps the connector switch in its secondary movement to an individual called line or to the first line in a P. B. X group. If a P. B. X group is called, an automatic stepping circuit causes the connector switch to hunt for an idle lin of the P. B. X roup. Relay @553 includes a selfinterrupting circuit which provides the hunting operation for the secondary movement or the selector switch and the hunting operation for the idle line of the P. B. X group.

In more detail, when the control relay unit is seized through the associated line finder 365, line relay Silt operates over the subscribers loop. Dial tone is immediately sent back to the substation A over the following path: D. T. terminal, contacts 33i, conductor are, relay 361'}, conductor +L, line finder Std, and substation A. Relay 3% operates and contacts 382 make to operate relay 3H Relay Sit operates and contacts 3H prepare a locking circuit for relay 3550. Contacts 352 prepare the pulsing circuit to drive magnet 53%. Contacts 3H3 make to operate release magnet 518 of the selector over the following path: ground, contacts 313, conductor 32%, conductor 4653, release magne and batter, Contacts 314 make to ground the C lead to hold the line finder 38 i and line equipment operated. Contacts 345 make to prepare a locking circuit for relay tit. Contacts 3h? close to prepare a lock ng circuit for busy relay tilt. Contacts 31! prepare a locking circuit for relay est. Contacts 358 prepare a circuit for slow-to-release relay .328.

The drive magnet of the selector steps the switch in the primary movement and is pulsed over the following circuit: battery, drive magnet 5 35, S. O. N. contacts 5H, conductor till, conductor 398, contacts 353, conductor 359, contacts dill, contacts #251, conductor 355, contacts 3%, contacts tit, conductor 32?, contacts did, conduotor 325, contacts conductor 325, conductor 59%, S. O. N. contacts 558 and ground. The drive magnet 538 is pulsed in response to contacts 38! operating as relay 3% repeats the incoming dial impulses.

The first series of dialled impulses is transmitted by substation A which interrupts the operating circuit of relay 38%? and thereby steps the drive magnet as; through the above-described circuit. After the first step primary off-normal P. O. N. contacts 52:! make to prepare a circuit for operating the shift magnet 5 S and P. O. N. contacts 525 break;

Relay S28 operates during each series of pulses over the following path: battery, relay 32%, contacts 315, contacts 333 ground. Contacts 32 make to operate slow-to-operate relay 3% at the completion of each series of pulses in multiple with relay 3H2. Contacts 323 make to operate relay 3228. Contacts 322 break to open the circuit to busy relay Contacts 3.2% break to open the circuit between the lower winding of relay 3&8 and the shift magnet Relay 435 operates over the following path: battery, relay 538, conductor contacts 323 and ground. Contacts $3 .5 break to remove dial tone from the substation A and substitutes direct ground at contacts 3-93 for grounded dial tone. Contacts 533 compete the locking circuit for relay 430 through contacts 3 l i.

The selector wipers 536, 537, 539, 55! and 5&2 are advanced to the selected dialled level and relay 3 35, shortly after dialling, operates over the following path: battery, relay t lt, contacts 32- 4, contacts 352 and ground. Contacts 3&2 close to operate relay 4 :8 in series with the shift magnet EGS of the selector. Contacts 343 complete a circuit from the lower Winding of relay Mt to the EC wiper 561 of the selector. Contacts 345 complete a circuit from test relay E66 to the C wiper 5 12 of the selector.

Relay M9 operates in series with the shift magnet SGE over the following path: battery, upper winding of relay are, conductor 33?, contacts 332, contacts 3&2, conductor contacts cs2, contacts 49:1, contacts lit, conductor 32d, 5. O. N. contacts 555?, P. O. N. contacts 526, shift magnet 56% and ground. Contacts 46! mak to operate slow-to-release step relay 159. Contacts M2 prepare a holding circuit for relay 3 26. Relay 32? releases a short time after the completion of the first series of pulses.

Shift magnet "35) operates an contacts 555 and 5&2 make to prepare the drive magnet for stepping the selector wipers in the secondary movement across the bank contacts of the dialled level. Contacts 58% and 595 break to keep the circuit to the wipers open. Contacts 5&5 close to maintain shift magnet 5% in series with the upper winding of relay ids. Contacts Eli! prepare a circuit to the cam springs and contacts 509 prepare a circuit to the ah trunks busy meter ATB MTR. The selector is now prepared to hunt an idle connector.

Relay 4% operates over the following path: battery, relay 45%, contacts at 5, contacts 565, com tacts 452, and ground on contacts 353. The selector is of. the battery searching type. Relay 65E! operates and the selector switch wipers 536", 531, 538, 539, 45! and 452 are stepped to the first bank contact of the selected hank level by operating the drive magnet 53% over the following path: battery, drive magnet 53%, contacts 592, conductor e87, conductor 3&8, contacts 353, conductor 309, contacts 683i, contacts ll I, contacts 555, conductor 305, contacts 3i2, conductor 32?, contacts M4, contacts 355, conductor 325, conductor 498, contacts 546 and ground. Secondary off-normal contacts of the selector operate in response to the first step in the secondary movement, contacts SH opening the original energizing circuit to magnet 536 now operated over contacts 502, contacts 518 disconnects ground from conductor 408 which conductor, however, is grounded at contacts 545 of operated shift magnet 580, and at contacts opens th original circuit of magnet 55!) now maintained at contacts 555. Self-interrupting contacts 452 of relay 450 open after each operation of relay "35$ and then close automatically to again complete an operating circuit for relay 455. This operation continues until the wipers 53$, 53?, 538, 535, 54!, 532, encounter an idle connector. Contacts 45! complete the pulsing circuit to the drive magnet 536 during the hunting operation of the secondary movement.

Assuming now that all of the connector switches accessible from this selected level are busy, then the selector wipers continue to step as described until cam springs CS of the selector operate. Cam springs 52? completes a circuit from ground contacts 5539 for operating the alltrunk-busy meter (not shown) over conductor ATB MTR. Cam contacts 526 complete a circuit for operating test relay 4% as follows: ground, winding of test relay 55e, conductor 356, contacts 345 and 42L cam contacts 526, contacts 59'! and through resistance R3 to battery. At contacts ib! relay 4% opens the circuit to the interrupter pulsing relay 450 to prevent its reoperation, at contacts 362 opens the series circuit of relay 44c and shift magnet 560 to cause their release. Relay etc at contacts ts: opens a further point in the self-interrupting circuit of relay 450 and at contacts 442 opens the circuit to relay 345.

In the selector switch shift magnet 50a releases when its circuit is opened and at contacts 582 opens a further point in the circuit to drive magnet 536. At contacts 595 shift magnet Eiiii opens a further point in its own circuit and at contacts 599 opens the all trunk meter circuit. At contacts 551 shift magnet 501i opens the circuit to test relay 459 which releases and at contacts 538 prepares a circuit for operating busy relay 455. The release of relay 459 is without effect at this time because relay 440 at contacts 4M has opened the circuit to relay 45c and because the series circuit to relay 54B and shift magnet 50!] is open at contacts 506 and 5H).

When relay 345 releases, as a result of the release of relay 440, a circuit is completed for operating busy relay 400 as follows: from ground, contacts 568, cam contacts 525, conductor 424, contacts 42!, 344, 322 and 4&6, winding of busy relay 4051 to battery. Busy relay 4% operates and at contacts 4G5 completes a locking circuit for itself to grounded contact 5H5 before opening its energizing circuit at contacts 406. At contacts ell! relay 455 opens a further point in the stepping circuit to drive magnet 536 and at contacts 4% opens a further point in the circuit to shift magnet 500. At contacts 492 busy relay substitutes grounded busy tone source for direct ground at contacts 403, to transmit a busy tone to the callin subscriber. The circuit for transmitting busy tone to the calling subscriber may be traced from grounded busy tone source over the busy conductor B. T., contacts 482, 432, and through the lower winding of line relay 3% and over the ca ling loop to the calling subscriber.

When th calling subscriber hears the busy tone and replaces his receiver the switch train will be released in a manner to be described more fully hereinafter.

When an idle connector is found, C wiper 542 encounters a battery potential on the C bank contact of the idle connector and test relay 46B operates over the following path: ground, relay 450, conductor 353, contacts 345, conductor 3 39, contacts 42!, conductor 424, contacts 515, Wiper 542, contacts 554, resistance 5 and battery. Relay 46B operates and contacts is: open the operating circuit to relay 459 to prevent further stepping in the secondary movement of the selector switch. Contacts 352 break to release shift magnet 5M and also relay ite. Relay 45d releases and contacts 45: break to open the pulsing circuit for drive magnet 53s to stop the secondary movement. Contacts 452 make to prepare an operating circuit for relay 45D, and contacts 453 break to further open the operating circuit for relay 440 through the lower winding.

Relay 440 releases and contacts Mi break. to further open the operating circuit of relay 450. Contacts 442 break to release relay 345i.

Relay 340 releases and contacts 344 make to connect relay 450 to the C wiper and contact 344 opens the circuit to test relay $55 which releases. Contacts 363 break to further open the operating circuit to relay 445 through the lower winding. Contacts 342 break to further open the operating circuit to the upper winding of relay 44D and shift magnet 56!].

Shift magnet 5&0 releases and contacts Efii and 593 make to extend a connection to the connec tor switch. Contacts 5% make to complete a circuit to operate the release magnet 530 cf the connector switch. Contacts 595 make to prepare a circuit to operate the shift magnet 52s of the connector. Contacts sec prepare a circuit to the cam springs. Relay 46c releases in response to the release of relay 345.

Release magnet 535 of the connector switch operates over the following path: battery, magnet sec, wiper 538, contacts 5H3, contacts 5%, conductor 469, conductor 326, contacts tit and ground.

The system is now prepared for the second series of digital impulses dialled from substation A and repeated by relay see through the contacts 36!. The interruptions of the operating circuit to relay soc will provide the repeating of the dialling impulses to the drive magnet 5H5 of the connector switch over the following path: battery, drive magnet 5w, S. O. N. contacts 528, wiper 536, contacts 5| contacts Edi, conductor 561, conductor 368, contacts 353, conductor 359, contacts 45], contacts M i, conductor pulsing contacts Sill, contacts tit, conductor contacts 4M, conductor 328, contacts 355, conductor 325, conductor 488, contacts 5%, contacts 5I2, Wiper 531, S. O. N. contacts 52.5 and ground. The switch steps in the primary movement and the connector wipers are now advanced opposite the dialled level.

Slow-to-release relay 320 operates again during the dialing of the second series of pulses over the following path: battery, relay sec, contacts 3H8, contacts 303 and ground. Relay 329st contacts 32i maintains the circuit to relay and shift magnet 520 open during the primary movement and at contacts 322 maintains the circuit to busy relay its open during the primary and secondary movements of the connector.

Slow-to-operate relay S it operates upon the completion of the second series of pulses before relay 328 releases over the following path: battery, relay 340, contacts 324, contacts are and ground.

In response to the first step in the primary movement of the connector switch P. O. N. con

'15 tacts 333 make to prepare an operating circuit to shift magnet i523, and P. C. N. contacts 5313 open to remove battery from C wiper Relay 53 at this time does not operate in series with the shift magnet 52%) of the connector, since battery is connected thereto.

Relay see operates before relay 3Z9 releases and contacts connect the shift magnet 529 in series with the upper winding of relay Md without efiect. Contacts 3&5 connect the lower winding of relay in series with the C wiper 5%. Contacts 345 connect relay its in series with the C wiper 5 35. A short time after the dialing of the second series of pulses, relay 328 releases.

Relay are releases and contacts 32! close to prepare a circuit placing the lower winding of relay 353 in series with the shift magnet 52B of the connector. Contacts 3E2 close to prepare the circuit to relay Contacts 323 break, but it has no effect on relay are since the circuit is looked over a previously described path. Contacts 32% break to release relay 3&3.

'Relay 3% role and contacts 3 5i complete a circuit placing the lower winding of relay 5338 in series with the shift magnet sec of the conhector.

Relay 33G operates in series with shift magnet 52!! of the connector over the following path: ground, lower winding of relay 33G, conductor 323, contacts lit, conductor contacts 3st,

contacts 33!, conductor 335, contacts 462, con-v tacts te l, contacts G18, conductor contacts 365, contacts bi l, wiper 539, P. O. N. contacts 533, magnet 52%) and battery.

Shift magnet 52c operates and contacts 5 opens the circuit to wiper 5 13 and contacts 5 prepare a circuit for operating magnet 555 to step the connector switch wiper across the banks of the dialled level in the secondary movement. Contacts 353 opens the circuit to wiper 5% and contacts 548 make to prepare an operating circuit for drive magnet did during the secondary movement.

Relay sec operated over a previously described path and contacts 33! complete a circuit between the lower winding of relay 33d and the shift magnet 528 so that when the relay 32G operates during the third series of dialing pulses, relay 328 will not interrupt the circuit connecting the lower winding of relay etc in series with the shift magnet 529. Contacts 332 break to open the circuit to the upper winding of relay see. Contacts 333 make to prepare a circuit to relay sic.

lhe is no prepared for the dialing of the third series of dialing impulses. Relay 3'69 repeats the digital impulses to operate the drive magnet 5h} to step the connector wiper in the secondary movement across the bank contacts of the dialled level. Drive magnet Sic is operated over the following path: battery, drive magnet Elia, contacts 522, wiper 535, contacts 51 1, contacts 59!, conductor dill, contacts 353, conductor 36%, contacts Ni, contacts 1H, conductor 385, contacts 38!, contacts 312, conductor 32?, contacts did, conductor 325, contacts 355, conductor 325, conductor cos, contacts 533, contacts 5l2, wiper 531, S. O. N. contacts 529 and ground.

The connector wipers take the first step and S. O. N. contacts 528 and 529 break. After the first step, the drive magnet 565 is operated over the following path: battery, magnet 5H3, contacts 522, wiper 535, contacts 5H, contacts 58!, conductor 407, contacts 353, conductor 389, contacts 49!, contacts dil, conductor 355, contacts Sill, contacts 3l2, conductor 32?, contacts 414, con- '16 ductor 326, contacts 355, conductor 325, conductor s88, contacts 503, contacts 562, wiper 337, contacts 548 and ground.

During the dialing of the third series of pulses relay 320 operates over the following path: battery, relay 320, contacts 3l3, contacts 303 and ground. Relay 32c operates and contacts 32! break, but they have no effect on the operation at this moment. Contacts 322 open the operat ing circuit to relay sec. Contacts 323 have no effect on the operation at this moment, since relay 400 is looked through contacts 5.33. Contacts 325 complete an operating circuit for slowto-oporate relay 3&8 which relay, however, due to its slow to operate characteristics, does not fully operate until shortly after the termination of the pulsing period.

Relay see operates at the completion of the dialing of the third series of impulses before relay 323 releases. Contacts 345 prepare the circuit to the testing relay use. Contacts 3:33 prepare a circuit to the lower winding of relay 44a.

The connector is now prepared to test for an idle subscribers line.

If the line is busy, relay @153 will not operate because of the ground connection connected to the bank contact engaged by wiper 54E. Relay 329 releases shortly after the completion of the third series of pulses. Contacts 32c break and relay 3% releases.

Relay 34c releases and contacts 343 complete a circuit to operate busy relay M30. Contacts 343 open a circuit between the lower winding of relay 44%! and the EC wiper ME.

Relay 4!!!! operates over the following path: battery, relay sou, contacts 105, conductor 35?, contacts 322, contacts 344, conductor 34%, contacts 42!, conductor 424, contacts 5L5, wiper 5 32, contacts 533, and ground on the bank contact of the called line engaged by C wiper 5M3. Make-before-break contacts 365 lock relay till! over the following path: battery, relay 358, contacts 405, conductor 358, contacts 3H5 and ground. Contacts I134 break to release the shift magnet 52c and relay 333 over a path previously described for operating the same. Contacts 502 make and contacts 493 break to send busy tone to the calling party over the following path: B. T. conductor, contacts 492, contacts 432, conductor 3E9, relay 3313, +L conductor, line finder 354 and substation A. Contacts Gill open the pulsing circuit to drive magnet 5 l E3. After receipt of busy tone the calling subscriber may release the connection by replacing his receiver in a manner to be described hereinafter.

If the called line is idle, test relay 1553 operates before relay 359 releases over the following path: ground, relay 4'59, conductor 3.55, contacts 345, contacts 42!, conductor 42%, contacts 515, wiper 542, contacts 333, C wiper 54c and through the winding of the B. C. O. relay of a called line to battery.

Relay 48!! operates and contacts at! open the operating circuit of relay 459. Contacts 352 open to release shift magnet 528 and relay Con-- tacts 463 open the circuit to the lower winding of relay Mil. Contacts 464 make to operate rolay M9 before contacts 333 open. Relay 33S and shift magnet 529 release in response to the operation of the test relay 460 and the shift magnet at contacts 521 and 523 prepares the ringing circuit.

Relay All! operates over the following path: battery, relay sic, conductor 3M, contacts 83.3,

17 conductor 348, contacts 464 and ground. Contacts M3 lock relay 4!!) over the following path: battery, relay 410, contacts M9, conductor 346, contacts 315 and ground. Contacts M8 and M further open the circuit to shift magnet 520 and relay 330. Contacts 4!1 make to connect the lower winding of relay 335 to the positive conductor. Contacts 420 make to ground the C lead to hold the called line busy. Contacts 4l5 make to ground the positive talking conductor, and contacts 4|3 make to connect ring back tone condenser 543 to the calling line. Contacts 4!2 make to connect the interrupted generator to the called line over the following path: interrupter generator, conductor INT.GEN., relay 356, conductor 365, contacts M2, contacts 40!, conductor 339, contacts 352, conductor 303, conductor 401, contacts 50!, contacts 5! I, wiper 536, contacts 52!, contacts 53! and wiper 543 over the called line and through the ringer at the called station back to wiper 544, contacts 532 and 523, wiper 531, contacts M2 and 503, conductor 403, contacts 355 and 4 5 to ground. Contacts 42! break to release relay 460 over a path previously described for operating the same. Relay 460 releases and contacts 464 break to open the original operating circuit of relay 4H3, contacts 463 make to prepare the circuit connecting the lower winding of relay 440 to the EC Wiper 545. Relay 320 releases as previously described and contacts 324break'to release relay 340. Contacts 345 break to further open the operating circuit for relay 45!). Ring back tone current will pass through condenser 549 to produce ring back tone to substation A over an obvious path.

When the called party anscwers, a direct current path is closed by the switchhook in the called substation to operate relay 350 overthe previously traced ringing circuit. Relay 350 operates and contacts 352 and 354 close to complete the talking circuit from substation A to the called line. Contacts lock relay 350 over the following path: battery, relay 350, contacts 35!, contacts 3!! and ground. Contacts and 355 break to remove the ringing current and ring back tone from the talking conductors.

loop as follows: ground, lower winding of relay 330, contacts 4.!1, 354, 533 and 5!2, wiper 531,

contacts 523 and 532, wiper 544, the called sta-' tion loop to wiper 543, contacts 53! and 52!, wiper 536, contacts 5! I 56 t, and 352, and through the upper winding of relay 330 to battery. Talk- -ing battery is fed through the windings of relay 330 to the called line and the talking battery for the calling line is fed through the windingsof line relay 300.

After conversation and when the called subscriber replaces his receiver relay 330 releases.

When the calling party replaces his receiver,

relay 300 releases'and contacts 302 break to release relay 3 l0.

Relay 3!!! releases after an interval and contacts 3! break to open the locking circuit to relay 350, contacts 3i3 break to de-energize release magnet 556 to release the selector switch and to de-energize release magnet 536 to release the connector switch. Contacts 3 !4 break to release the line finder 364 and the associated line equipment. Contacts 3i 5 break to open the looking circuit of relay 4!!! to release relay 4I0. Contacts 3!1 break to open the locking circuit to relay 430 to release. the same. Contacts 3!6 break to open the locking circuit of busy :z elay Back bridge relay 330 now operates over the called line 18 400 in case relay 400 is operated. Relays 350, M0 and 430 release and the de-energization of release magnets 5! 0 and 530 cause the release of the selector and connector switches. The system is now prepared for further operation.

The foregoing disclosure had assumed that an individual line was called. Referring now to Figure 8 and assuming that a P. B. X group had been called, EC wiper 545 and C wiper 546 of the connector would. haveencountered conditions as shown in Figure 8 in response to stopping on the first trunk line of a P. B. X group of lines.

It should be remembered that relay 340 operates at the end of a third series of pulses over previously described path. Since EC wiper 345 is now in engagement with the first EC bank contact of a P. B. X group contacts 345 make to operate relay 440 over the following path:v ground on EC bank contact of the first P. B. X trunk line, wiper345, wiper 54!, conductor 422, contacts 463, conductor 339, contacts 343, conductor 338, lower winding of relay 440 and battery. Relay 460 does not operate if the first P. B. X trunk line is busy since the C wiper 546 encounters a ground instead of battery.

Relay 440 operates and contacts 44! make to operate relay 450 over the following path: battery, relay 450, contacts 44!, contacts 46!, contacts 452 and ground. Contacts 442 close to maintain relay 340 operated over the following path: ground, contacts 442, conductor 36!, relay 340 and battery.

Relay 450 operates and contacts 45! close to automatically step the connector across the trunks of the P. B. X group in search of an idle line over the following path: battery, magnet 5!6, contacts 522, wiper 536, contacts 5!!, contacts 50!, conductor 401, conductor 308, contacts 353, conductor 309, contacts 40!, contacts 4!! contacts 45!, conductor 304, contacts 3!2, conductor 321, contacts 4!4, conductor 326, contacts 355, conductor 325, conductor 408, contacts 503, contacts 5! 2, wiper 531, contacts 548 and ground. Each time relay 450 operates it interrupts its own operating circuit at contacts 452 and operates contacts 45! of the pulsing circuit to operate the drive magnet 5l6.

In case the first trunk line in a P. B. X group is idle or if an intermediate trunk line is idle when the C wiper 546 engages its associated bank contact, relay 460 will operate in the following manner: ground 460, conductor 356, contacts 345, conductor 34!], contacts 42!, conductor 424, contacts 5! 5, "C wiper 542, contacts 533, C wiper 546, and battery through the winding of the B. C. O. relay of the idle trunk line.

Relay'460 operates and contacts 46! break to release relay 450 and prevent further hunting by opening the pulsing circuit. Contacts 462 open the circuits to relay 330 and shift magnet 520 which release as previously described and contacts 463 open the opening circuit to relay 440 which releases. Contacts 464 complete the circuit for operating relay 4I0.

Relay 4!!) operates over the following path: ground, contacts 464, conductor 348, contact 333, conductor 341, relay M0 and battery. Relay 4!0 operates and causes a switch through to the idle trunk line in a manner previously described in reference to a call to a direct line.

If an intermediate'line is not idle, relay 440 is.

maintained operated by ground extending through the associated resistance to the EC contacts, as shown in Figure 8 connected to the intermediate lines. The EC contact individual to the last trunk line in. the P.- B. Xgroup is not connected to ground with the result that when the connector wipers arev stepped into engagement with the bank contacts of this last trunk line relay 440 releases since there is not any ground thereon to maintain it operated. Contacts MI break to prevent further operation of relay 45!) thereby stopping further hunting and contacts 442 releases relay 340. Relay 400 operates in case the last trunk line is busy in response to the release of relay 340 in a manner previously described to send busy tone back to the calling party. Upon the calling party replacing his handset, the switch train and relays release in a manner previously described.

. In the two embodiments shown and described it will be noted that only one selector switch is shown. The common control unit circuits and the switch circuits are such that any desired number of intervening selector switches may be used between the common control unit and the final connector switch, so that this system could be used for larger networks. It should be noted that the shift magnets in the selector switches are connected through their windings to ground while the shirt magnets in the final connector switches are connected throughtheir windings to battery. In the common control units the stepping control relays, such as relays I30 in Fig. 2 and relay 840 in Fig. 5, are connected to battery and at times in series with the shift magnets and when these relays are operatedthe drive magnets of the switches are automatically stepped by self-interrupting relays, such as relays I2ll and 459 in a manner apparent from the foregoing descriptions. Therefore, if the selectors shown have access to succeeding selectors instead of connectors the succeeding selectors would be operated in the same manner as described for'the selectors shown. For example, if the selector in Fig. 3 has access to a. succeeding selector instead of the connector shown, the selector in Fig. 3 seizes the idle succeeding selector in response to the C wiper 225 encountering a negative battery potential connected to the C bank contact of the succeeding selector. This battery potential extends from battery through a resistance; similar to resistance R, P. O. N. springs, similar to P. 0. N. springs ZIB and to the C" bank contact of this succeeding selector. The test relay I60 in the commoneontrol unit is operated from this battery potential to stop the further stepping operations of the succeeding selector in the same manner as explained for the selector of Fig. 3.

After seizure the succeeding selector is then directivelyoperated in its primary movement in response to the succeeding dialled digit and when relay 5i! energizes shortly after this digit the'circuit for operating relay I30 and the shift magnet of the succeeding selector is completed to ground through the winding of the shift magnet of the succeeding selector. This circuit may be traced from ground through the winding of the shift magnet of the succeeding selector over P. O. N. contacts, similar to P. O. N. contacts 2 I 5, S. O. N. contacts, similar to S. O. N. contacts 2M, bank contact and wiper 235, contacts 2I2 and 203, and thence over conductor IIlIv and contacts I85, 45, II2, IHI, 5|, and 43, and winding of relay I30 to battery. In the same manner as described for the selector of Fig. 3 the succeeding selector is automatically stepped in its secondary movement in response to the operation of relay I30 and operations of relay I and switchesthrough when an. idle trunk is round. Further successive selectors may be seized and operated as just described. and when the last switch in the switch train, for example, the connector of Fig. 3, has been operated in its primary movement the circuit is completed to shift magnet 226. As previously described, the shift magnet 225 is connected to battery and when relay 50 operates it connects shift magnet 220 to relay I39 which is also connected to battery and therefore neither the shift magnet 220 or relay I operate. However, when relay 50 releases the lower winding of relay 4!] is energized in series with shift magnet 220 as previously described.

The operation of relay at this time places the common control unit so as to directively operate the connector in responseto the last dialled digit, as previously described. The common control unit is therefore advised by the operation of relay Ml when battery, instead. of ground, is encountered through the shift magnet that the last switch has been reached and that no automatic stepping is to takeplace. unless a P. B. X group is called.

.In the second embodiment shownv in Figs. 4, 5 and 6 succeeding selectors may also be included in the switch train before the connector switch is reached. All of the shift magnets of the succeeding selectors would be connected to ground to control the operation of relay 440, which in turn controls the self-interrupting relay 450 to automatically step the succeeding selectors in an obvious manner apparent from the foregoing descriptions. Since shift magnet 520 of the connector is connected to battery, relay 640 will not operate when the common control unit is connected to the connector switch and relay 330 is operated in series with shift magnet 520 as previously described to enable the connector to be directively operated. in. its secondary movement in response to the last dialled digit.

While there has been described what is at present considered to be the preferred embodiment of the invention, it is to 'be understood that various modifications may be made thereof, and is contemplated in the appended claims to cover all such modifications within the true spirit and scope of the invention.

What is claimed is:

1. In a telephone system, a first switch hava primary and a secondary movement, a plurality of. groups of second switches accessible to said first switch, each of said. second switches having a primary and a secondary movement, a plurality of groups of lines accessible to each of said second switches, a control unit associated with said first switch, means for seizing and releasing said control unit, transmitting means in said control unit for transmitting impulses to said first switch, a first drive magnet in said first switch for operating said first switch in its primary movement to select a group of said second switches, means whereby said first drive magnet is operated responsive to impulses transmitted by said transmitting means, a first shift magnet in said first switch for changing over the operation of said first switch from its primary to its secondary movement, circuit means in said control unit for operating saidv first shift magnet, relay means for operating said circuit means, means whereby said relay means is operated responsive to the termination of said impulse trans znission by said transmitting means and after said primary movement, testingv means in said control unit. control means in said control unit 21 for reoperating said first drive magnet to opcrate said first switch in its secondary movement in a testing operation, means whereby said testing means is successively connected to said secnd switches in said selected group in said testing operation, means whereby said control means is operated responsive to the operation of saidcircuit means, means in the first idle one of said second switches for operating said testing means, means whereby said means for operating said testing means is effective responsive to the connection of said testing means to said first idle second switch, means for disabling said control means to stop the testing operation and the secondary movement of said first switch to seize said idle second switch, means whereby said disabling means is operated responsive to the operation of said testing means, a first release magnet in said first switch, release control means in said control unit for perating said first release magnet, means whereby said release control means is operated responsive to the seizure of said control unit, switch connecting means for connecting said control unit to said seized idle second switch, means whereby said switch connecting means is completed responsive to the operation of said first release magnet and said seizure of said second switch, a second drive magnet in said idle seized second switch for operating said second switch in its primary movement to select a group of said lines, means whereby said second drive magnet is operated responsive to subsequent impulses transmitted by a reoperation of said transmitting means, a second shift magnet in said seized second switch for changing over the operation of said second switch from its primary to its secondary movement, other circuit means in said control unit for operating said second shift magnet, means whereby said other circuit means is operated responsive to another operation of said relay means, means whereby said relay means is again operated by said transmitting means after said subsequent impulse transmission and after said second switch primary movement, said second drive magnet again operated in response to a further reoperation of said transmitting means to operate said second switch in its secondary movement, means whereby a particular line of said selected group of lines is selected in response to said secondary movement, a second rel ase magnet in said seized second switch, means including said release control means for operating said second release magnet, means whereby said means for operating said second release magnet is completed in response to the seizure of said second switch by said first switch, line connecting means for connecting said second switch to said selected line, means whereby said line connecting means is completed in response to the operation of said second re lease magnet and to said seizure of said line, means for restoring said first and second release magnets to release said first and second switches, and means whereby said restoring means is responsive to the release of said control unit.

2. In a telephone system, a first switch, a plurality of groups of second switches accessible to said first switch, a plurality of groups of lines accessible to each of said second switches, a control unit associated with said first switch, transmitting means in said control unit for transmitting impulses to said switches, switch directing means in said first switch for operating said first switch to select a group of said second switches, means whereby said switch directing means is operated responsive to impulses transmitted by said transmitting means, a test circuit in said control unit including first and second test relays, circuit controlling means in said control unit for reoperating said switch directing means to operate said switch in a testing operation, means whereby said test circuit is successively connected to said second switches in said selected group in response to said testing operation, means for operating said circuit controlling means, means whereby said means for operating said circuit controlling means is operated responsive to the termination of said impulse transmission by said transmitting means, means in the first.

idle one of said second switches for operating the first of said test relays, means whereby said means for operating said first test relay is effective responsive to the connection of said test circuit to said first idle one of said second switches in said selected group, means for disabling said circuit controlling means to stop the testing operation of said first switch and to seize said idle second switch, means whereby said disabling means is operated responsive to the operation of said first test relay, drive means in said seized second switch for operating said seized switch to select a group of said lines accessible thereto, means whereby said drive means is operated responsive to subsequent impulses transmitted by said transmitting means, means for reoperating said circuit controlling means, means whereby said means for reoperating said circuit controlling means is operated responsive to the termination of said subsequent impulse transmission by said transmitting means and said selection of said group of lines, said drive means reoperated in response to a reoperation of said circuit controlling means to operate said second switch in a testing operation, means whereby said test circuit is connected to successive lines of said selected group in response to said last mentioned testing operation, means in the first idle line of said selected group for again operating said first test relay, means whereby said means for again operating said first test relay is efiective responsive to the connection of said test circuit to said first idle line, said disabling means again operated by said reoperation of said first test relay for disabling said circuit controlling means to stop the testing operation of said second switch and to seize said idle line, said of groups of lines accessible to each of said second switches, a control unit associated with said first switch, transmitting means in said control unit for transmitting impulses to said first switch, a first drive magnet in said first switch for operating said first switch in its primary movement to select a group or" said second switches, means whereby said first drive magnet is operated in response to impulses transmitted by said transmitting means, a first shift in said switch for changing over the operation of said first switch from its primary movement to its secondary movement, circuit means in said control unit for operating said first shift magnet to, condition said first switch. for its secondary movement, means whereby said, circuit means is efiective responsive to the termination of said impulse transmission by said transmitting means, a test circuit in said control unit including first and second test relays connected in series, pulsing means in said control unit for reoperating said first drive magnet to operate said first switch in its secondary movement in a testing operation, means whereby said test circuit is successively connected to said second switches in said selected group in response to said secondary movement, means whereby said pulsing means is operated responsive to the operation of. said circuit, means, means in the first idle one of said second switches foroperating said first test relay, means whereby said means forv operating said first test relay is effective responsive to the connection of said test. circuit to said first idle second switch in, said selected group, means for disabling said impulsing means to stop the testing operation and said secondary movement to seize said idle second switch, means, whereby said disabling means is, operated responsive to the operation of said first test relay, a second drive magnet in said seized second switch for operating said second switch in its primary movement to select a group of said lines, means whereby said second drive magnet is operated responsive to a subsequent reoperation of said transmitting means, a second shift magnet in said second switch for changing over the operation of said second switch from its primary to its secondary movement, means in said control unit for operating said second shift magnet, means whereby said means for operating said second shift magnet is operated responsive to said subsequent reoperation of said transmitting means and said primary movement of said second switch, said second drive magnet again operated in response to a further reoperation of said transmitting means for operating said second switch in an initial secondary movement to select a particular line of said selected group of lines, other means in said control unit for re-enabling said pulsing means, means whereby said other means is operated responsive to said selection of said particular line, said second drive magnet reoperated in response to the operation of said re-enabled pulsing means for operating said second switch in a further secondary movement in a line testing operation, means whereby said test circuit is successively connected to said lines in said selected group in response to said line testing operation, means associated with the first idle one of said lines in said selected group for operating both said first and second test relays, means whereby said means for operating both said test relays is effective responsive to the connection of said test circuit to said first idleone of said lines in said selected group, said disabling means again operated by said first test relay for disabling said pulsing means to stop the line testing operation and said further secondary movement to seize said idle line, means for completing a switch through operation from said control unit to said seized idle line, and means whereby said last mentioned means is operated responsive to the operation of said second test relay.

In a telephone system, a switch having a primary and a secondary movement, a plurality of groups of second switches accessible to sa d first switch, a control unit associated with said switches, transmitting means in said control unit for transmitting a series. of pulses to said first switch, a drive magnet in said first switch for stepping said first switch in the primary movement to select a particular group of second switches, means whereby said drive magnet is operated responsive to the transmission of a series of pulses by said transmitting means, a first relay in said control unit, means associated with said control unit for operating said first relay, means whereby said means for operating said first relay is responsive to the completion of a transmitted series of pulses by said transmitting means, a second relay in said control unit, a shift magnet in said first switch for causing said first switch to change from a primary movement to a secondary movement, means for operating said shift magnet and said second relay in series, means whereby said shift magnet operating means is effective responsive to the operationv of said first relay, a test circuit in said control unit, a pulsing circuit in said control unit for operating said drive magnet to cause said first switch to step in the secondary movement, means whereby said test circuit is successively connected to said second switches in the selected group in response to said secondary movement, means for completing said pulsing circuit, and means whereby said completing means is operated responsive to the operation of said second relay.

5. In a telephone system, a first switch having a primary and a secondary movement, a plurality of groups of second switches accessible to said first switch, a control unit associated with said switches, transmitting means in said control unit for transmitting a series of pulses to said first switch, a drive magnet in said first switch for stepping said first switch in the primary movement to select a particular group of second switches, means whereby said drive magnet is operated responsive to the transmission of the series of pulses transmitted by said transmitting means, a first relay in said control unit, means associated with said control unit for operating said first relay, means whereby said first relay operating means is operated responsive to the completion of the transmitted series of pulses by said transmitting means, a second relay in said control unit, a shift magnet in said first switch for causing said first switch to change from a primary movement to a secondary movement, means associated with said control unit for operating said shift magnet and said second relay in series, means whereby said shift magnet operating means is efiective responsive to the operation of said first relay, a test circuit in said control unit, a pulsing circuit in said control unit for operating said drive magnet to cause said first switch to step in the secondary movement, means whereby said test circuit is successively connected to said second switches in the selected group in response to said secondary movement, means for completing said pulsing circuit, means whereby said completing means is operated responsive to the operation of said second relay, a circuit in said control unit for maintaining said first relay operated, and means whereby said circuit is com pleted responsive to the operation of said second relay.

6.1m a telephone system, a first switch having a primary and a secondary movement, a plurality of groups of second switches accessible to said first switch, a control unit associated with said switches, transmitting means in said control unit for transmitting a series of pulses to said first switch, a drive magnet in said first switch for stepping said first switch in the primary movement to select a particular group of second switches, means whereby said drive magnet is operated responsive to the transmis sion of the series of pulses transmitted by said transmitting means, a first relay in said control unit, means for operating said first relay, means whereby said first relay operating means is operated responsive to the completion of the transmittted series of pulses by said transmitting means, a second relay in said control unit, a shift magnet in said first switch for causing said first switch to change from a primary movement to a secondary movement, means associated with said control unit for operating said shift magnet and said second relay in series, means whereby said shift magnet operating means is efiective in response to the operation of said first relay, a testing relay in said control unit, a pulsing circuit in said control unit for operating said drive magnet to cause said first switch to step in the secondary movement, means whereby said testing relay is successively connected to said second switches in the selected group in response to said secondary movement, means for completing said pulsing circuit, means whereby said completing means is operated in response to the operation of said second relay, a circuit in said control unit for maintaining said first relay operated, maintaining means whereby said maintaining circuit is completed in response to the operation of said second relay, means in the first idle one of said second switches in said selected group for operating said testing relay, means whereby said means for operating said testing relay is effective in response to the connection of said testing relay to said first idle second switch, means for stopping the stepping of said first switch in its secondary movement by opening said pulsing circuit and for releasing said second relay, means whereby said stopping means is operated in response to the operation of said testing relay, and said maintaining circuit opened by said maintaining means in response'to the release of said second relay to release said first relay. 7

7. A telephone system as claimed in claim t including a circuit for operating said testing relay, and means whereby said circuit for operating said testing relay is prepared in response to the operation of said first relay.

8. In a telephone system, a first two-movement switch, a plurality of groups of second two-movement switches accessible to said first switch, a plurality of groups of lines accessible to each of said second switches, acontrol unit associated with said switches, transmitting means associated with said control unit for transmitting three series of pulses, a first drive magnet in said first switch for stepping said first switch selectively in the primary movement to select a particular group of second switches, means whereby said first drive magnet is operated responsive to the first series of pulses transmitted by said transmitting means, a first shift magnet insaid first switch for changing over the operation of said first switch from'a primary to a secondary movement, a first relay in said control unit, means in said control unit for operating said first relay, means whereby saidfirst relay operating means is operated resiaonsivexto-the completion of the first series of pulses transmitted by said transmitting means, a second relay in said control unit,

means in said control unit for operating said first shift magnet and said second relay in series, means whereby said shift magnet operating means is effective in response to the operation of said first relay, a pulsing circuit in said control unit for reoperating said first drive magnet to cause said first switch to step in the secondary movement, means whereby said first switch is connected to successive second switches in said selected group in response to said secondary movement, means for completing said pulsing circuit, means whereby said pulsing circuit completing means is operated in response to the operation of said second relay, means for stopping the secondary movement of said first switch, means whereby said stopping means is operated in response to said first switch connecting with the first idle one of said second switches in said selected group to select said idle second switch, a second drive magnet in said selected second switch for stepping said second switch selectively in the primary movement to select a particular group of lines accessible to said selected second switch, means whereby said second drive magnet is operated in response to said second series of pulses transmitted by said transmitting means, a second shift magnet in said second switch for changing over the operation of said second switch from a primary movement to a secondary movement, said first relay operating means reoperating and restoring said first relay in response to the completion of the second, series of pulses transmitted by said transmitting means, a third relay in said control unit, means for operat ing said third relay in series with said second shift magnet, means whereby said means for operating said third relay and second second shift magnet is operated in response to the restoration of said first relay, and said second drive magnet again operated by said transmitting means in response to the third series of pulses to step said second switch in the secondary movement to select a particular line of said selected group of lines.

9. In a telephone system, a first two-movement switch, a plurality of second two-movement switches accessible to said first switch, a plurality of groups of bank contacts accessible to each of said second switches, a control unit associated with said switches, transmitting means in said control unit for transmitting three series of pulses, a first drive magnet in said first switch for stepping said first switch in the primary movement to select a particular group of second switches, means whereby said first drive magnet is operated by said transmitting means in response to the first series of pulses, a first shift magnet hi said first switch for changing over the operation of said first switch from a primary to a secondary movement, a first relay in said control unit, means in said control unit for operating said first relay, means whereby said first relay operating means is operated responsive to the completion of the first series of pulses transmitted by said transmitting means, a second relay in said control unit, means in said control unit for operating said second relay in series with said shift magnet, means whereby said second relay and said shift magnet operating means is operated in response to the operation of said first relay, a pulsing circuit in said control unit for operating said first drive magnet to operate said first switch in the secondary movement, means whereby said first switch is connected to successive secondswitches in the selected group in aeeaaei response to said secondary movement, means for completing said pulsing circuit, means whereby said pulsing circuit completing means is operated in response to the operation of said second relay, means for stopping the secondary movement of said first switch, means whereby said stopping means is operated responsive to said first switch connecting with the first idle one of said second switches in said selected group to select said idle second switch, a second drive magnet in said selected second switch for operating said second switch selectively in the primary movement to select a particular group of bank contacts accessible to said selected second switch, means whereby said second drive magnet is operated by said transmitting means in response to the second series of pulses, a second shift magnet in said second switch for changing over the operation of said second switch from a primary to a secondary movement, said first relay reoperated and restored by said first relay operating means in response to the completion of the second series of pulses by said transmitting means, a third relay in said control unit, means in said control unit for operating said third relay in series with said second shift magnet, means whereby said third relay and said second shift magnet operating means is operated in response to the restoration ofsaid first relay, said second drive magnet again operated by said transmitting means to step said second switch in the secondary movement in response to the third series of pulses for selecting abank contact of said selected group of bank contacts, an alternating current relay in said control unit, means for extending a connection from said selected bank contact to said alternating current relay, a source of alternating current potential on said selected bank contact effective for operating said alternating current relay over sa d extended connection, means whereby said pulsing circuit is completed in response to the operation of said alternating current relay, and said second drive magnet operating in response to the completion of said pulsing circuit to step said second switch to the next bank contact.

' l0. In a telephone system, a first two-movemerit switch, a plurality of groups of second twornovement switches accessible to said first switch, a plurality of groups of bank contacts accessible to each ofsaid second switches, a control unit associated with said switches, transmitting meansin said; control unit for transmitting three series of pulses, a first drive magnet in said first switch for stepping said first switch in the pri- "irymov menrm select a particular group of secondswitches, means whereby said first drive magnets'o'perated by said transmitting means in response: tothe first series of pulses, a first shift magnet in said first switch for causing said first switch to change from a'primary to a secondary movement, a first relay in said control unit, means for operating said first relay, means whereby said first relay operating means is operated by said transmitting means responsive to the completion of the first series ofpulses, a SEC-1 ond relayin said control unit, means in said control unit for operating said first shift magnet in series with said second relay, means whereby said first shift'magnet and said second relay operating means is operated in response to the operation of said first relay, a pulsing circuit in said control unit for operating said first drive magnet to cause said first switch to step in the secondary movement, means whereby said first switch is connected to successivev second switches in the selected group in 'response to said secondary 28 movement, means for completing said pulsing circuit, means whereby said pulsing circuit completing means is operated in response to the operation of said second relay, means for stopping the secondary movement of said first switch, means whereby said stopping means is operated in response to said first switch connecting with the first idle one of said second switches in said selected group to select said idle second switch, a second drive magnet in said selected second switch for stepping said second switch in the primary movement to select a particular group of bank contacts accessible to said selected second switch, means whereby said second drive magnet is operated by said transmitting means in response to the second series of pulses, a, second shift magnet in said second switch for causing said second switch to change from a primary movement to a secondary movement, said first relay reoperated and restored by said first relay operating means in response to the completion of the second series of pulses by said transmitting means, a third relay in said control unit, means in said control unit for operating said third relay in series with said second shift magnet, means whereby said third relay and said second shift magnet operating means is operated in response to the restoration of said first relay, said second drive magnet reoperated by said transmitting means to directively step said second switch in the secondary movement in response to the third series of pulses to select a contact of said selected group, means for extending a connection from the selected bank contact to said second relay, a source of potential on said selected bank contact of said selected group of bank contacts, means whereby said source of potential is effective over said connecting means to reoperate said second relay, said pulsing circuit again completed in re-- sponse to the reoperation of said second relay, and said second drive magnet operating in re-, sponse to the reoperation of said pulsing circuit to step said second switch to the next bank con-1 tact.

11. In a telephone system, an automatic switch, a first and second group of bank contacts accessible to said switch, said second group of bank contacts having idle and busy bank contacts, an alternating current relay, a test relay, means in: cluding said switch for connecting said alternating current relay to one contact of said first group of bank contacts, means including said switch for connecting said test relay to one contact of said second group of bank contacts, a source of alternating current potential connected to said one contact of said first group of bank contacts, means whereby said alternating current source is effective to operate said alternating current relay in response to the connection of said alternating current relay to said one contact of said first group, a stepping circuit associated with said switch for stepping said switch, means whereby said switch is connected with the succeeding bank contacts of said first and second groups of bank contacts in response to said stepping operation, means whereby said stepping circuit is completed in response to the operation of said alterhating current relay, means for operating said test relay, means whereby saidtest relay operating means is efiective in response to said switch connecting with an idle bank contact of said sec: ond group of bank contacts, means for opening said stepping circuit to stop said switch, and means whereby said last means is operated by said test relay in response to the operation of said test relay. 

